Method of manufacturing miniature incandescent lamps

ABSTRACT

In the manufacture of a single ended subminiature incandescent lamp, a helically coiled wire filament is bent into a hairpin shape and is mounted on a temporary support while the mandrel on which the wire has been coiled is still within the filament. The filament is then inserted into acid in order to dissolve the mandrel at the apex portion of the filament. The filament is then sealed within a glass envelope.

United StatesfPatent n91 Kopelman METHOD OF MANUFACTURING MINIATUREINCANDESCENT LAMPS [75] Inventor: Bernard Kopelman, Magnolia,

Mass.

[73] Assignee: GTE Sylvania Incorporated,

Danvers, Mass.

[22] Filed: June 7, 1973 [21] Appl. No.: 367,819

[52] US. Cl. 29/25.l5, 313/271 511 int. Cl. H01j 9/18 58 Field or Search29/25.14, 25.15; 25.17,-

[56] References Cited UNITED STATES PATENTS H1937 Zabel. 29/25. 13 X[451 Feb. '4, 1975 2,359,302 Curtis 29/25.13 X 3,270,238 8/1966 Mosby313/279 3,356,884 12/1967 Decker et al. 313/212 3,594,884 7/1971 Siege]29/2513 X Primary Examiner-Roy Lake Assistant Examiner-James W. DavieAtt0rney, Agent, or FirmJames Theodosopoulos [57] ABSTRACT In themanufacture of a single ended subminiature incandescent lamp, ahelically coiled wire filament is bent into a hairpin shape and ismounted on a temporary support while the mandrel on which the wire hasbeen coiled is still within the filament. The filament is then insertedinto acid in order to dissolve the mandrel at the apex portion of thefilament. The filament is then sealed within aglassenvelope.

9 1 Claim, 4 Drawing Figures 1 METHOD OF MANUFACTURING MINIATUREINCANDESCENT LAMPS BACKGROUND OF THE INVENTION tric currenttherethrough. It especially relates to a method of making such lamps.

Subminiature lamps generally have a filament made of such fine wire thatit can only be seen and handled under magnification; thus the coil mountand assembly is done by hand with consequent high costs. The filament isgenerally made by helically coiling fine tungsten wire on a metalmandrel wire, several times its diameter, in a long continuous length.The coiling isheat treated to set it andis then cut into predeterminedfilament lengths. The mandrel is dissolved in acideither before or afterthe cutting step. The filament is then hand mounted on a suitablesupport and then sealed within a small glass envelope.

It is an object of'this invention to provide a method of manufacturingthat permits employment of auto-- matic assembly machines, therebyeliminating much of the hand work involved and thereby significantlyre-' ducing the cost of such lamp.

THE INVENTION helically coiled wire filament mountedon a temporarysupport.

FIG. 2 shows the filament immersed in acid to dissolve the mandrel IFIG. 3 shows the filament, still mounted on the temporary support,sealed into one end of a glass envelope.

FIG. 4 shows the finished lamp. T In the manufacture of a single endedsubminiature incandescent lamp in accordance with this invention, finetungsten wire 1, generally less than 1 mil diameter, is continuouslyhelically coiled on a wire mandrel 2 of larger diameter. The mandrel 2is made of a suitable metal, say, nickel, steel or molybdenum, that canbe dissolved in an acid that does not readily attack tungsten; In atypical case, the mandrel diameter is 3m 4 mils.

The coiling-with-mandrel can than be heat treated, if

necessary, to set the coiling and is then taken up on a spool in'a longcontinuous length. Next, the coiling is mounted on suitable apparatuswhere it is unreeled and cut into predetermined lengths. The cut lengthsare then bent into a hairpin or U shape and the ends thereof areattached to a suitable-temporary support 3. The fact that the mandrel 2is still within each cut length gives it sufficient bulk to permit thebending and attaching steps to be performed on automatic equip- .ment.The distance between legs of the bent-filament at the time of attachingto a suitable support 3 is small enough to permit the filament to beinserted into a tubular glass envelope 7 at the time of lamp assemblywhile the filament is mounted on said support 3. In a typical case, thedistance between legs-was about onesixteenth inch.

The support 3 on which the bent filament is mounted must be suitable tosupport the filament until the filament is sealed in a glass envelope 7.The support 3 may be a small flat piece of metal to which the ends ofthe bent filament are attached, as by welding. In such a case,themetalpiece remains outside the lamp envelope 7 and is removed afterthe filament is sealed in the blocks in which the legs of the bentfilament are clamped. The blockshold the filament through all operationsuntil after the filament is sealed in the lamp envelope.

After the bent filament has'been mounted on the support 3, the filamentis dipped in a suitable acid 4 to dissolve the mandrel 2. The mandrel 2is dissolved in at least the effective lighted length of thefilament butnot at theportions thereof which are part of the glass-tometalseal inthe finished lamp. Thus, the support 3 is not dipped into the acid4 andthe mandrel 2 is not dissolved in those portions of the filament legswhich are not immersed in the acid. The acid may be heated to reduce thetime for dissolving the mandrel to a few seconds. Typically,hydrochloric acid is used to dissolve a steel mandrel, while acombination of sulphuric and nitric acids is used to dissolve amolybdenum mandrel. After the filament is rinsed and dried, it is readyfor sealing in. a glass envelope 7. The filament is inserted into oneend of a small diameter glass tubing 5, typically, T] or Asinchdiameter, and the end of the tubing is heated to its-softening point.Thesoftened end is then squeezed on the legs of the filament to form ahermetic press seal 6. The temporary support 3 remains outside theenvelope 7 and is removed after the filament has been sealed in theenvelope.

In any case, however, it is important that mandrel 2 remain withinthat'portion of the filament which is embedded in the glass which formsthe glass-to-metal seal of the lamp envelope 7. The reasonfor this is tokeep the electrical resistivity of the conductors which are embedded inthe glass low, so that they will not overheat during lampoperationwhich, ifoverheating occurred, could crack the glass seal. Also, themandrel may aid somewhat in forming a hermetic seal during the pressoperation if the filament has highTPl (turns per inch) coiling.

After the filament has been inserted in one end of the glass tubing andsealed there, the tubing is exhausted through the other end and is thenheated and tipped off diameter to aid in tipping off the lamp.

In an example of a 5 volt 60 milliampere Tl lamp made in accordance withthis invention, the filament coiling consisted of 0.6 mil tungsten wirewound on 4 mil molybdenum wire at 650 TPI. The coiling was heat treatedat l,525C and then cut into lengths of 20 mm. Each cut length wasthenfolded in a hairpin and the ends thereofwere welded to a temporarysupport consisting of a rectangular piece of 4 mil nickel, about 6 mm by3 mm. The legs of the hairpin were about 1% mm apart. The apex of thehairpin was then inserted into heated sulfuricnitric acid to a depth ofabout 1 /2 mm for about 5 to 10 seconds to dissolve the molybde toexhaust and filling apparatus. The first end of the glass tubing washeated to its softening point and then pressed onto the legs of thefilament to form a hermetic press seal. If necessary, the interior ofthe tubing could be flushed with a suitable inert gas to preventoxidation of the filament during press sealing. The other end of theglass tubing was then heated and tipped off while a vacuum wasmaintained in the tubing. The temporary subminiature incandescent lamp,the steps which sequentially comprise: helically coiling a length offine tungsten wire on a mandrel wire having a larger diameten heattreating and cutting the coiling into predetermined lengths; bendingeach of the cut lengths into a hairpin shape; attaching the ends of thehairpin to a temporary support; dissolving part of the mandrel at theapex end of the hairpin; inserting'the hairpin into an open ended glasstube; heating and press sealing one end of the glass tube onto a portionof the legs of the hairpin, said portion still containing mandrel wire;sealing the other end of the glass tube and removing the temporarysupport to form a finished lamp.

' at 1: t 4: a:

1. In the process of manufacturing a single ended subminiatureincandescent lamp, the steps which sequentially comprise: helicallycoiling a length of fine tungsten wire on a mandrel wire having a largerdiameter; heat treating and cutting the coiling into predeterminedlengths; bending each of the cut lengths into a hairpin shape; attachingthe ends of the hairpin to a temporary support; dissolving part of themandrel at the apex end of the hairpin; inserting the hairpin into anopen ended glass tube; heating and press sealing one end of the glasstube onto a portion of the legs of the hairpin, said portion stillcontaining mandrel wire; sealing the other end of the glass tube andremoving the temporary support to form a finished lamp.